Tick-borne bacterial pathogens of humans cause significant morbidity and mortality throughout the United States and abroad. Lyme disease (LD), caused by Borrelia burgdorferi, is the most prevalent arthropod-borne disease of humans in the United States and many other countries throughout Europe and Asia. Tick-borne relapsing fever (TBRF), caused by Borrelia hermsii, is endemic in scattered foci throughout many regions of higher elevation in the western United States. Our work has focused on two areas: 1) to improve on the serodiagnosis of LD and TBRF fever by using recombinant DNA technology to clone genes of spirochetes that express proteins that induce specific and detectable antibody responses; 2) to examine how spirochetes adapt to their tick and mammalian hosts. This work requires that we maintain colonies of Ixodes scapularis and Ornithodoros hermsi, respective tick vectors of LD and TBRF spirochetes, and infect these ticks via a laboratory mouse - tick cycle. During the last year we investigated the distribution and activity of an enzyme, glycerophosphodiester phosphodiesterase (GlpQ) among species of Borrelia. Our hypothesis is that difference in the metabolism of glycerol-3 phosphate, a critical intermediate in phospholipid synthesis and glycolysis, is involved in differences in the pathogenicity of TBRF and LD spirochetes. Eighteen isolates of Borrelia including 14 species were assayed for GlpQ activity. No LD spirochetes had activity while all TBRF spirochetes did. Activity was observed in different phases of growth and was present with membrane fractions of B. hermsii. Genomic sequences identified 5 gene members of the glp locus in B. hermsii and B. turicatae. However, the genes coding for GlpQ and GlpT, a glycerol-3-phosphate transporter, were absent from 6 species of LD spirochetes. Mice immunized with GlpQ and challenged with B. Hermsii were not protected and relapsed with spirochetemias subsequent to the initial bacteremia. Future efforts will be directed at determining the fate of phospholipid components in the metabolism and growth of TBRF spirochetes. Work also continued of the genomic sequencing of two species of Borrelia and a cDNA library of the salivary glands of Ornithodoros hermsi was constructed to identify salivary components important in the transmission of TBRF spirochetes.